In a cellular communication system, a geographical region is divided into a number of cells each of which is served by base stations. The base stations are interconnected by a fixed network which can communicate data between the base stations. A mobile station is served via a radio communication link from the base station of the cell within which the mobile station is situated.
A typical cellular communication system extends coverage over an entire country and comprises hundreds or even thousands of cells supporting thousands or even millions of mobile stations. Communication from a mobile station to a base station is known as the uplink, and communication from a base station to a mobile station is known as the downlink.
The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a mobile station in a cell to communicate with a mobile station in any other cell. In addition, the fixed network comprises gateway functions for interconnecting to external networks such as the Internet or the Public Switched Telephone Network (PSTN), thereby allowing mobile stations to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc.
Currently, the most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile communication (GSM). GSM uses a technology known as Time Division Multiple Access (TDMA) wherein user separation is achieved by dividing frequency carriers into 8 discrete time slots, which individually can be allocated to a user. Further description of the GSM TDMA communication system can be found in ‘The GSM System for Mobile Communications’ by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.
Currently, 3rd generation systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) technology. Both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) techniques employ this CDMA technology. In CDMA systems, user separation is obtained by allocating different spreading and scrambling codes to different users on the same carrier frequency and in the same time intervals. In TDD, additional user separation is achieved by assigning different time slots to different users similarly to TDMA. However, in contrast to TDMA, TDD provides for the same carrier frequency to be used for both uplink and downlink transmissions. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS). Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
In a 3rd generation cellular communication system, the communication network comprises a core network and a Radio Access Network (RAN). The core network is operable to route data from one part of the RAN to another, as well as interfacing with other communication systems. In addition, it performs many of the operation and management functions of a cellular communication system, such as billing. The RAN is operable to support wireless user equipment over a radio link of the air interface. The RAN comprises the base stations, which in UMTS are known as Node Bs, as well as Radio Network Controllers (RNC) which control the base stations and the communication over the air interface.
The RNC performs many of the control functions related to the air interface including radio resource management and routing of data to and from appropriate base stations. It further provides the interface between the RAN and the core network. An RNC and associated base stations are known as a Radio Network Subsystem (RNS).
3rd generation cellular communication systems have been specified to provide a large number of different services including efficient packet data services. For example, downlink packet data services are supported within the 3GPP release 5 specifications in the form of the High Speed Downlink Packet Access (HSDPA) service. A High Speed Uplink Packet Access (HSUPA) feature is also in the process of being standardised. This uplink packet access feature will adopt many of the features of HSDPA.
In accordance with the 3GPP specifications, the HSDPA service may be used in both Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode.
In HSDPA, transmission code resources are shared amongst users according to their traffic needs. The base station or “Node-B” is responsible for allocating and distributing the resources to the users, within a so-called scheduling task. Hence, for HSDPA, some scheduling is performed by the RNC whereas other scheduling is performed by the base station. Specifically, the RNC allocates a set of resources to each base station, which the base station can use exclusively for high speed packet services. The RNC furthermore controls the flow of data to and from the base stations. However, the base station schedules transmissions to the mobile stations that are attached to it, operates a retransmission scheme, controls the coding and modulation for transmissions to and from the mobile stations and transmits (for HSDPA) and receives (for HSUPA) data packets from the mobile units.
HSDPA and HSUPA seek to provide packet access techniques with a relatively low resource usage and with low latency.
Specifically, HSDPA and HSUPA use the following techniques in order to reduce the resource required to communicate the data thereby increasing the capacity of the communication system:                Adaptive Coding and Modulation. The coding and modulation schemes may dynamically be selected to be optimised for the current radio conditions thereby providing effective link adaptation. For example, in HSDPA, the 16QAM higher order modulation may be used to increase throughput for users in favourable radio conditions whereas the less efficient but more robust QPSK modulation may be used at less favourable radio conditions.        Retransmission with soft combining. HSDPA and HSUPA use a retransmission scheme known as a Hybrid-Automatic Retransmission reQuest (H-ARQ) scheme wherein retransmissions are soft combined with previous transmissions in order to achieve an efficient communication. The H-ARQ scheme is typically operated at a higher block error rate for individual transmissions in order to increase efficiency, but the final block error rate after soft combining is similar to the block error rate for pre-HSDPA systems.        Fast scheduling is performed at the base station. This allows scheduling to be sufficiently fast to dynamically follow radio condition variations. For example, when more than one mobile unit requires service, the base station may schedule data to the mobile stations experiencing favourable radio conditions in preference to the mobile stations experiencing less favourable conditions. Furthermore, the resources allocated and the coding and modulation applied to transmissions to mobile stations may be highly tailored to the current radio conditions experienced by the individual mobile station.        
HSDPA and HSUPA furthermore use the following techniques in order to reduce the delay (latency) associated with the data communication:                Short transmission time intervals. Specifically, data transport blocks are sent to the transmitter at frequent time intervals thereby allowing for transmissions and retransmissions to be transmitted with a minimum of delay.        Scheduling and retransmission functionality located at the base station. This may reduce the delay associated with scheduling and retransmissions as control and data need not be communicated between the RNC and the base station.        Increased capacity. The reduced resource usage and associated increased capacity in itself reduces the delay incurred by buffering of data as a higher data capacity may provide a higher throughput and thus reduced queue sizes.        
However, despite these techniques, the performance is not optimal. Specifically, in conventional systems an operating point is selected to provide an acceptable capacity and latency performance. Although such an operating point may provide acceptable performance in general, it is not optimal for many situations and may in particular result in a relatively high latency. For example, although an increased capacity may reduce queuing delays, it may also increase other delays such as the delay associated with retransmissions. Therefore, in order to achieve a sufficiently high capacity, the retransmission delays may frequently result in a latency which is higher than desired.
For example, in order to achieve a sufficiently high capacity, it is important to transmit data packets at a sufficiently low transmit power. The associated queuing delay is thus reduced. However, this results in an increased block error rate and thus an increased number of retransmissions being required for successful communication. As the delay before a retransmission occurs is substantial, this may substantially increase the resulting average delay of transmitting data packets.
Hence, an improved system for communication would be advantageous and in particular a system allowing for increased flexibility, improved performance, reduced latency and/or increased capacity would be advantageous.